1. Field of the Invention
The present invention relates to an inner spring to be fitted inside furniture including chairs and sofas, and bedding including mattresses for beds.
It is noted that the terms "furniture" and "bedding" used herein are intended to embrace any seats used in, for example, trains and motorcars and these terms are used, in the broad sense without being limited to the examples above.
2. Description of the Prior Art
In a general type inner spring producing apparatus, expensive and highly resilient, oil tempered, wire rods are used to enhance resilience or repulsion and durability of coil springs. This is because coil springs made of general wire rods present problems in resilience and durability.
However, the oil-tempered wire rods are strong in resilience, thus presenting a problem in that the coil springs are produced at much expense in time and labor. In addition, since those coil springs are low in production and yet the wire rods are expensive, a problem of increasing manufacturing costs is presented.
In general, a row of cased coil springs is produced by the method that each of the coil springs, which are produced continuously by the coil spring producing apparatus, is inserted in order between a non-woven fabric or sheet as folded in half The non-woven fabric or sheet is sealed. In this method, each individual cased coil spring cannot be changed in diameter.
To allow the row of coil springs to have partially different resilient forces (spring rates) with respect to a direction of a length L of, for example, a mattress for a bed by the conventional method, it is required that a variety of rows of inner springs be prepared. Each row of inner springs has encased coil springs which have different diameters. Rows of inner springs encasing coil springs of required diameters for required resilient forces are selected so as to be properly arranged in the widthwise direction W and are connected in order. Thus, the mattress of the bed as shown in FIG. 12 is formed.
With this conventional method, a plurality of apparatuses specialized for required diameters of the wire rods are needed to produce a plurality of rows of cased coil springs including coil springs having different diameters. In addition, the lengths of the rows of cased coil springs must be disadvantageously changed in accordance with variations in width W of the mattresses.
To solve these problems, the applicant has previously proposed an inner spring producing apparatus in U.S. Pat. No. 5,740,597 (Japanese Laid-open Patent Publication No. Hei 9-173673). This apparatus which is designed so that the coil springs are energized to be heated so as to be hardened, and are sequentially inserted into cylindrical casings having a predetermined number of spring casing portions which are continuously formed in parallel.
However, the coil springs produced by the inner spring producing apparatus previously proposed by the applicant as well as by the above-described general type ones are all uniform in resilient force (spring rate). Because of this, the mattress, in which a plurality of rows of conventional coil springs, each having a uniform resilient force, are connected, becomes uniform in resilient force over the whole area. However, the distribution of weight through the mattress is not uniform, as illustrated in FIG. 12, which shows that the mattress is weighted lightly at its portions 34, 35 corresponding to the head and legs, but is weighted heavily at its portions 36 corresponding to the trunk. Thus, the mattresses has a disadvantage of not being suitable for the non-uniform distribution of weight.
One possible approach for allowing the coil springs to have different resilient forces to suit such a non-uniform distribution of weight is to use wire rods having different diameters for the coil springs. But, in this case, a plurality of coil spring producing apparatuses specialized for required diameters of individual wire rods are needed for producing the coil springs. This presents a further problem of needing a large space in a work-shop for installation of machines, storage of a variety of wire rods and the like.
In addition, each of the coil springs separately produced by the coil spring producing apparatuses must be adequately controlled to be accurately held in position in the spring casing portions of the cylindrical casings. Thus, further problems of needing much time and labor to reduce productivity and of being high in probability of miss-casing arise. Consequently, quality and reliability are reduced.
Another approach is that various kinds of springs of different diameters aligned in rows (the coil springs cased in each row of inner springs are all equal in diameter) are produced by a plurality of inner spring producing apparatuses. Proper rows of inner springs encasing coil springs of required diameters are selected and are connected so that they can be arranged in the widthwise direction W of the mattress, as shown in FIG. 13. This approach can produce resilient forces partially different with respect to the length L of the mattress, but this still has a disadvantage that the lengths of the rows of cased coil springs must be changed in accordance with variations in width of the mattresses.
Thus, various problems in addition to the above-described problems are created.
In order to solve the problems mentioned above, the applicant previously proposed "inner springs fit inside furniture and bedding and a producing method therefor" in U.S. patent application Ser. No. 09/126,446 (Japanese Patent Application No. Hei 10(1998)-58449).
The inner spring producing apparatus previously proposed by the applicant in the U.S. Pat. application Ser. No. 09/126,446 is provided with a energizing type hardening device which is structured so as to control the time for the coil spring to be energized for the hardening to provide varied resiliency for the coil springs. This proposed inner spring producing apparatus is epoch-making in that it can provide coil springs having various resiliencies while producing the coil springs for the inner springs, so as to provide partly varied support with respect to a lengthwise direction L of a mattress. This advantageous effect can never be produced by the conventional type of inner spring producing apparatus with no energizing type hardening device. In other words, it can never be produced without newly introducing the inner spring producing apparatus having the energizing type hardening device.